BH perturbation theory dates back to the early '60's when Regge, Wheeler [1]} and then Zerilli [2]} studied linearised wave equations around Schwarzschild geometryAccording to K. S. Thorne [3]}, `singularity' at that time meant the `horizon', the original emphasys was on 'wormhole', a term coined by J. A. Wheeler ... as `black-hole', or `hair, or `S-matrix'.
